1. Field of the Invention
The present invention relates to a laser microscope for scanning a specimen under inspection with laser light.
The present invention also relates to a pattern checking apparatus using a laser microscope, and more particularly to a pattern checking apparatus, in which a periodically formed pattern can be checked at a high speed. It should be noted that in the present specification, the term laser microscope means a microscope in which laser light emitted by a laser light source is projected onto a specimen, light reflected by the specimen or transmitted through the specimen is received by a photo-detector by means of a pin hole or a frame defining an aperture of a light receiving area, and an image of the specimen is formed by processing an output signal from the photo-detector. Further, a term confocal optical system means an optical system in which a light beam emitted from a point-like light source is projected onto a specimen as a light spot and light reflected by the specimen or transmitted through the specimen is received by a photo-detector by means of a pin hole.
2. Statement of the Related Art
There has been known to check or detect a defect in a substance having a plurality of patterns such as LSI, wafer with a pattern and photomask, by illuminating a whole surface of the substance with illumination light in a two-dimensional manner and light reflected by the specimen is received by a two dimensional CCD array. In this known defect detecting apparatus, a whole surface of the substance to be checked is illuminated, an image of the substance is picked-up by the two-dimensional CCD array, and output signals from respective light receiving elements of two-dimensional CCD array are read out in a serial manner by means of a transfer circuit, and a defect is detected from the thus read out output signals.
There has been also known to detect defects by utilizing a laser microscope. Such a laser microscope has been known from, for instance U.S. Pat. No. specification 4,736,110. In case of detecting defects by using the laser microscope, a light source is formed by a laser diode, laser light is deflected two-dimensionally by means of two beam deflecting devices such as vibrating mirror, light reflected by a specimen is received by a photodetector such as a photo-multiplier, and defects are detected on the basis of an output signal from the photo-multiplier. In order to perform a high speed scanning, it has been also known to use an acoustic-optical deflection element and a vibrating mirror. Since the above mentioned laser microscopes constitute the confocal optical system, it is possible to attain a high resolution, and therefore the laser microscopes are particularly suitable for detecting defects of a specimen having fine patterns such as LSI.
In the above mentioned defect detecting apparatus, in which a two-dimensional image of the specimen is picked-up by means of the two-dimensional CCD array and defects are detected from output signals of respective light receiving elements of the CCD array, the output signals from a number of light receiving elements of the CCD are read out in a serial manner, and therefore a relatively long time is required for reading out the CCD array and thus a defect detection requires a long time. In case of the laser microscope using two vibrating mirrors, a longer time is required for scanning a whole surface of a specimen, and therefore a longer time is required for performing the defect detection. In the laser microscope using a combination of an acoustic-optical deflection element and a vibrating mirror, a high speed scanning can be effected, and thus a whole specimen can be checked within a relatively short time. However, the high speed scanning is limited by a fact that only one laser beam is used. Moreover, in order to drive the acoustic-optical element, a circuit for generating a high frequency electric signal is required and not only the construction of the microscope becomes complicated but also a manufacturing cost becomes high.